Communications messages transmitted over a transmission medium (which may be electrical, acoustical, optical, or electromagnetic) are subjected to various delays and distortions caused by channel characteristics such as dispersion, fading, multipath, etc. Accordingly, one of the functions of a communications receiver is to remove distortions in the message caused by such channel characteristics.
Pseudo random noise (PRN) modulated signals are generally known as "spread spectrum" signals because they utilize time sequences to synthesize a signal having wideband frequency attributes. The use of spread spectrum techniques to transmit messages through noisy or hostile RF channels is well known.
In spread spectrum communications, a narrowband message is convolved with a wideband noise source, such as a pseudo random noise ("PRN") generator, or the amplified output of a noise diode, or any such randomized signal source.
For consistency in terminology, the following definitions relating to the spread spectrum process are used herein: A "chip" is the basic value of the randomized signal source for a duration of time equal to the reciprocal of the code clock frequency ("chipping rate"). A "bit" is used herein to refer to the basic value of the message for a duration of time equal to the reciprocal of the data clock ("bit rate"). The "chip-to-bit ratio" ("C/B") refers to the ratio of the code clock frequency to that of the data clock. An increased C/B results in an increased transmitted signal bandwidth relative to the message bandwidth and a higher process gain for the spread spectrum system. The terms "PN" and "PRN" and "m-sequences" refer to maximal length pseudo (random) noise, which is a deterministic signal with noise-like characteristics, usually generated with (linear or non-linear) feedback shift registers or toggle registers.
Spread spectrum systems typically utilize a pseudo random noise (PN) generator clocked at a high chipping rate relative to the data bit clock. Code generators are typically constructed with a feedback shift register generator configured to provide a maximal length code which is used to spread the message, and an identical code generator at the receiver, which, after proper synchronization, will de-spread the received message. Synchronization typically involves a three step process, first obtaining code clock synchronization and then code phase synchronization, and finally data clock synchronization. Until code clock and phase synchronization is achieved, no message information is recoverable; moreover, due to the code clock alignment criteria used to obtain code phase synchronization, such prior art systems intrinsically ignore message carrying signals which arrive by paths differing in delay by more than one chip. Moreover, in systems where the PN modified message is modulated onto a carrier, an additional initial synchronization step is required to regenerate a phase coherent replica of the carrier at the receiver.
Prior art spread spectrum systems also include transmitted reference systems in which the reference is transmitted on a separate RF frequency, using bandpass filters in the receiver to filter the reference signal from the bandspread message signal. Because there is no need for a second code generator in the receiver there is no need for any code clock, and code phase synchronization between the two code generators and the acquisition of the message is thus facilitated. However, separating the reference in frequency from the reference modulated message nullifies any potential capability to compensate for frequency selective fades and dispersion effects. Moreover, use of a modulated carrier in such transmitted reference systems still requires a locally regenerated replica of the carrier.
In I-Q transmission schemes, the carrier comprises two orthogonal basis vectors, with a 90 degree (.pi./2) phase difference, such as sin(.theta.) and cos(.theta.). These two basis vectors meet the criteria of orthogonality and normality, since ##EQU1## and sin.sup.2 (.theta.)+cos.sup.2 (.theta.)=1. A linear combination of these two basis vectors will create an arbitrary vector in the space of signals. If the modulation changes the sign of the magnitude of the basis vector at data transitions, such a system is called "quadrature phase shift keying", or QPSK. If the system is constrained so that the data transitions applied to each basis vector do not occur at the same time, such a system is called "offset-QPSK". If the timing of the transitions is further constrained, as is typically done, so that the transitions on one basis vector occur at the mid-bit time of the data on the other basis vector, such a system is called ".pi./2-offset QPSK".
FIG. 1 is a simplified representation of a generic prior art spread spectrum transmission and reception system 10. A carrier signal CARRIER.sub.T is modulated in the transmitter section 12 with a wideband signal WB.sub.T which is composed of pseudo noise PN.sub.T convolved with low rate data signal DATA.sub.IN. The resultant modulated carrier signal MOD.sub.T is fed to a linear amplifier and flat phase filters 14.sub.T and transmitted. In the receiver section 16, similar linear amplification and filtering 14.sub.R is applied to the received modulated carrier signal MOD.sub.R and the received data DATA.sub.OUT is recovered by convolving the filtered received modulated carrier signal MOD.sub.R ' with a local carrier CARRIER.sub.R which has been made phase coherent with the transmitter carrier CARRIER.sub.T, and with a local replica PN.sub.R of the original pseudo noise signal PN.sub.T, which has been code aligned and made coherent with the transmitter chip clock CHIP CLOCK.sub.T. In principle, the processes in the receiver 16 need not be performed in any particular order--the pseudo noise PN.sub.R may be used to modulate the output of the receiver local oscillator LO which then is convolved with the incoming signal, or the pseudo noise PN.sub.R may direct multiply the incoming signal either before (MOD.sub.R '), or after (not shown) the signal has been mixed with the local carrier CARRIER.sub.R.
FIG. 2 is a block diagram of a typical prior art spread spectrum receiver 16, showing various subsystems associated with carrier frequency and phase acquisition 21, code clock and phase acquisition 22, data bit clock synchronization 23, and data output 24.
FIG. 3 is a simplified representation of a typical prior art "transmitted reference" spread spectrum system 10', in which the PN code is sent on a separate carrier frequency CARRIER.sub.T2, from the carrier frequency CARRIER.sub.T1 used to transmit the PN coded message (WB.sub.T1). In such a system, the transmitter 12' sends RF signals (MOD.sub.T1, MOD.sub.T2) out on two frequencies (f1, f2), and it is important to maintain good linearity from the time the two RF signals MOD.sub.T1, MOD.sub.T2 are added (summer 25) until they are convolved (mixer 26) in the receiver 16'. This means that all the transmitter and receiver amplifiers and filters (14.sub.T1, 14.sub.T2, 14.sub.R1, 14.sub.R2, 18.sub.R1, 18.sub.R2) must have good linearity and high dynamic range. Furthermore, while not explicitly shown in FIG. 3, the prior art dual frequency transmitted reference receiver 16' must be provided with all of the carrier acquisition circuitry (21) required to generate two local carriers (CARRIER.sub.R1, CARRIER.sub.R2), each locked in phase with a respective RF signal carrier, thereby potentially doubling the carrier acquisition circuitry associated with the single local carrier in the receiver of FIG. 2. The code clock and code phase acquisition circuitry is avoided, however, due to the use of a "transmitted reference" pseudo noise signal PN.sub.R ' rather than having to synchronize a local pseudo noise generator to produce a replica PN.sub.R (FIG. 2) of the original pseudo noise signal PN.sub.T.
The carrier acquisition circuitry required in the receiver of prior art spread spectrum systems not only adds to the expense and complexity of the receiver, but also requires a relatively long interval at the start of transmission for carrier acquisition before any information can be received, and is prone to error if the carrier is subject to rapid changes in frequency because of signal path induced frequency errors and/or Doppler effects. Moreover, because the reference signal and the encoded message were not subjected in prior art spread spectrum transmission schemes to identical signal path distortion effects, any signal path distortion effects had to be filtered out in the receiver prior to using the undistorted reference to de-convolve the original message from the undistorted encoded message. Accordingly, such prior art transmitted reference systems were not suitable for particularly wideband applications having a high chipping rate which is a substantial fraction (at least 10%) of the carrier frequency. In that regard, it is noted that two non-overlapping spread spectrum signals that together occupy a substantial portion of the LF (100 kHz to 2 MHz), HF (2 MHz to 30 MHz), or VHF (30 MHz to 200 MHz) region will be subject to significantly different multipath effects.